Living animals generate electrical potentials which, when collected, detected and analysed, can be used for a variety of purposes. For example, synchronous neural activity in a live animal or human brain produces electrical potentials that can be detected at the surface of the scalp with conductive electrodes. These detected potentials can then be used in a wide variety of clinical applications, particularly diagnostic applications.
It is known to collect these electrical potentials generated by living animals through the application of passive electrodes applied to the skin of the animal. These electrodes consist of a conductive surface or pad that is coupled or adhered to the skin of a subject. The operation of the conductive pad is often facilitated by the additional application of a conductive substance, such as gel, between the skin and the electrode. The conductive pad of the electrode is connected to a lead wire which, in turn, is electrically coupled to an amplifier. The length of the lead wire is typically in excess of 1 m (usually from approximately 1 m to 2.5 m) and electrically connects the amplifier (housed in a signal processing device) and the electrode. The amplifier amplifies the difference in electric potentials between a signal electrode and a reference electrode, both of which are affixed to the subject (human or animal). The amplifier is typically housed together with some signal processing device which, typically, is also adapted to record and analyse any detected electrical potentials which have been amplified by the amplifier. Unfortunately, this known arrangement of the electrode, lead wire and amplifier has significant shortcomings, particularly for the following reasons.
Unlike typically well known electrical potentials in common use in other industries and other areas of activity, the electrical potentials generated by living animals are often very small in amplitude—often in the millivolt, microvolt, or even nanovolt range. As a result, these electrical potentials are easily “drowned out” or lost due to noise from the electrical potentials generated by other items in the vicinity of the subject (e.g., lighting, the signal processing device, other equipment, etc.). That is, the differential electric potentials of interest in most applications (often smaller than 1 microvolt) are usually smaller than the electrical noise that is detected by the amplifier when no signal is present.
Significant sources of electrical noise which will often be detected by the amplifier are caused by the plurality of time-varying and time-invariant electromagnetic fields that are often present in a test environment where the electrode-lead wire-amplifier arrangement is employed. These time varying electromagnetic fields are inductively and capacitively coupled to the lead wire that carries the signal from the electrode to the amplifier. Consequently, these time varying electromagnetic fields introduce noise onto the lead wire that will be detected and amplified by the amplifier. A second significant source of noise is motion artefacts; i.e., the noise induced in the lead wire as it moves through a static (i.e., time-invariant) electromagnetic field.
To address these known shortcomings, efforts have been made to shorten the lead wire in an attempt to reduce noise. However, these efforts have had limited success. Amongst the problems with these efforts is that it is impractical in many applications to tether a subject (whether it is an animal or human) with a wire that is less than about 1 meter long to the amplifier.
Another measure to reduce noise that has had some success, albeit limited, is achieved with differential measurements since common mode noise, i.e. noise that is identically present in two wires, can be cancelled to a certain degree. Unfortunately, not all of the noise induced by the various electromagnetic fields is identical in both signal wires and, thus, some significant amount of noise will still be not cancelled and thus present in the recording system.
Additional efforts to reduce the effect of noise include conducting multiple tests and then averaging the results of these multiple tests. Unfortunately, conducting repeated tests in an attempt to eliminate or reduce any noise detected has the unwanted effect of significantly lengthening the testing process. Since it is often preferred that the subject remain still or, in some cases, unconscious, a lengthening of the testing process is quite undesirable especially when the test subject is a young child or animal.
Another shortcoming with the known electrode-based systems of measuring electrical potentials is the difficulty in determining whether the electrodes have been properly attached or affixed to the subject (e.g., animal or human), while proper attachment, as typically indicated by low electrical impedance between the electrodes, is crucial for the recorded signal-to-noise ratio. As a result, significant care must be taken by the clinician to properly attach these electrodes and then carefully monitor any potentials measured to assess whether the measurements are indicative of improper electrode attachment. If a clinician or other operator is of the opinion that at least one of the electrodes is improperly attached to the subject, a time consuming review of each electrode is necessary to determine which electrode is improperly attached to the subject. To overcome this time consuming process some clinical systems include impedance detection, i.e., a means for automatically detecting if an electrode is poorly connected with the skin of a subject. The accepted method of impedance detection (see for example U.S. Pat. No. 5,368,041) is to introduce a small-current signal to each electrode. The voltage from each electrode to ground is measured and is proportional to the impedance of the electrode. However, such an impedance-detection system requires additional circuitry and the introduction of another electrical current. This additional current and circuitry will be a further source of noise in any signal detected. Moreover, the additional circuitry increases the costs and complexity of the overall system.
Accordingly, a method and apparatus for the collection of electrical potentials which addresses, at least in part, some of the above-noted shortcomings is desired.